Apparatus and method for continuous lyophilization

ABSTRACT

A method and apparatus for the continuous sublimation of a substance includes cascading a material containing a substance capable of sublimation, such as water, between a plurality of trays vertically stacked within a processing zone provided within a processing chamber. A substantially atmospheric environment is maintained within the processing zone at a temperature whereby the substance sublimes forming a sublimate within the environment. The environment containing the sublimate is contacted with a drying agent such as a desiccant to maintain the environment whereby the substance sublimes at substantially atmospheric pressure and at the controlled temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/950,336, filed Nov. 19, 2010, which claims the benefit of thefiling date of U.S. Provisional Patent Application No. 61/284,068, filedDec. 11, 2009, entitled “Apparatus And Method For ContinuousLyophilization,”, and U.S. Provisional Patent Application No.61/355,282, filed Jun. 16, 2010, entitled “Apparatus And Method ForContinuous Lyophilization,” the disclosure of which is herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates in general to an apparatus and method forthe continuous lyophilization, also known as freeze-drying, of amaterial by sublimation under substantially atmospheric pressure.

Lyophilization is a dehydration process typically used to preserve aperishable material or make the material more convenient for transport.A common application for the lyophilization of perishable materialinclude pharmaceuticals and food products. Lyophilization overcomes twoof the common problems associated with conventional drying of materialby evaporation. The first problem is the inherent difficulty of removingwater completely using evaporation because most of the water is notdirectly exposed to the air. Second, the liquid water involved in theevaporation process significantly changes the shape, texture andcomposition of the material. This is a particular disadvantage in theprocess of drying material that it intended to be revitalized later.

The fundamental principle in lyophilization is sublimation which is anendothermic phase transition that occurs at temperatures and pressurebelow the triple point. At normal pressures, most chemical compounds andelements possess three different states at different temperatures. Inthese cases, the transition from the solid to the gaseous state requiresan intermediate liquid state. As shown in the phase diagram of FIG. 1,the boundary between gas and liquid runs from the triple point 10 to thecritical point 20. Freeze drying (arrow 30) brings the system around thetriple point, avoiding the direct liquid to gas transition seen inordinary drying (arrow 40). Sublimation occurs when the partial pressureof the compound or element in the surrounding atmosphere is less thanthe vapor pressure of the compound or element in the material beinglyophilized. Various apparatus and methods for lyophilization ofmaterial are known from U.S. Pat. Nos. 5,269,077; 4,561,191; 4,104,807;3,731,392; 3,484,946; and 3,462,849, the disclosures of which areincorporated herein by reference and copies enclosed herewith.

SUMMARY OF THE INVENTION

The present invention describes a method for the continuous sublimationof a substance, the method comprising cascading a material containing asubstance capable of sublimation between a plurality of trays verticallystacked within a processing zone provided within a processing chamber,maintaining a substantially atmospheric environment within theprocessing zone at a temperature whereby the substance sublimes forminga sublimate within the environment, and contacting the environmentcontaining the sublimate with a drying agent to maintain the environmentwhereby the substance sublimes at substantially atmospheric pressure andat the controlled temperature.

The present invention further describes a method for the continuoussublimation of a substance, the method comprising, transferring materialcontaining a substance capable of sublimation through a plurality ofprocessing zones each having a material support arranged in a verticalstack with a processing chamber, controlling an environment within theprocessing chamber at substantially atmospheric pressure and at atemperature whereby the substance sublimes to form a sublimate withinthe environment, and contacting the environment containing the sublimatewith a drying agent to control the partial pressure of the substancewithin the environment whereby the substance sublimes at thesubstantially atmospheric pressure at the temperature.

The present invention further describes a method for the continuoussublimation of a substance, the method comprising, supplying material inparticulate form containing a substance in a frozen state, the substancecapable of sublimation, cascading the material downwardly between aplurality of trays vertically stacked within a processing chambermaintained under substantially atmospheric pressure, controlling thetemperature of the environment within the processing chamber below 32°F. whereby the substance sublimes, controlling the partial pressure ofthe substance within the environment within the process chamber lessthan the vapor pressure of the substance in the material at least bycontacting the environment with a desiccant or a gas having a low dewpoint, whereby the substance continuously sublimes within the processingchamber.

The present invention further describes a method for continuouslyophilization by sublimation of a substance, the method comprisingfreezing a material containing a substance capable of sublimation,providing the frozen material in particulate form to a processingchamber, cascading the frozen material downwardly within the processingchamber between a plurality of rotating trays vertically stacked withinthe processing chamber, each of the trays forming a processing zone,maintaining a substantially atmospheric environment within each of theprocessing zones and at a temperature less than 32° F. whereby thesubstance sublimes forming a sublimate within the environment, andcontrolling the partial pressure of the substance within the environmentat each of the processing zones less than the vapor pressure of thesubstance in the frozen material.

The method further includes, wherein the environment is maintainedwhereby the partial pressure of the substance in the processing zone isless than the vapor pressure of the substance in the material.

The method further includes, wherein the temperature of the environmentis in the range of about 20-30° F.

The method further includes, wherein the drying agent is a desiccant isselected from the group consisting of lithium chloride, calciumchloride, calcium sulfate, phosphorous pentoxide, triethylene glycol,diethylene glycol, ethylene glycol, tetrathylene glycol, silica gel, andmolecular sieves, as well as other desiccants.

The method further includes, supplying the material to the processingchamber in particulate form.

The method further includes, wherein the material is supplied at atemperature of less than 32° F. whereby the substance is frozen.

The method further includes, recovering the sublimate from thedesiccant.

The method further includes, recovering the sublimate from theenvironment by condensing.

The method further includes, wherein the substance comprises water.

The method further includes, circulating the environment within theprocessing chamber by a plurality of fans within the processing chamber.

The method further includes, wherein the pressure in the processingchamber is maintained between about ±0.10 inches water.

The present invention further describes a method for continuouslyophilization by sublimation of water in a solid phase within amaterial, the method comprising freezing a material containing water toa temperature below 32° F., whereby the water is present in a solidphase, providing the frozen material in particulate form in the range ofabout 8-200 mesh to a processing chamber, cascading the frozen materialin particulate form downwardly within the processing chamber between aplurality of rotating trays vertically stacked within the processingchamber, each of the trays forming a processing zone, maintaining asubstantially atmospheric environment in the range of about ±0.10 incheswater within each of the processing zones and at a temperature in therange of about 20°-30° F. whereby the water in a solid phase sublimesforming a sublimate within the environment, controlling the partialpressure of the water within the environment at each of the processingzones less than the vapor pressure of the water in solid phase in thefrozen material by at least contacting the environment with a desiccant,heating the environment after contacting the environment with thedesiccant, and recycling the heated environment to the processingchamber.

The present invention further describes an apparatus for continuoussublimation of a substance, the apparatus comprising a processingchamber containing a substantially atmospheric environment, theprocessing chamber having a plurality of trays vertically stackedtherein for cascading therethrough a material containing a substancecapable of sublimation to form a sublimate at a predeterminedtemperature, and a desiccant in fluid communication with the processingchamber for maintaining the partial pressure of the substance within theenvironment within the processing chamber less than the vapor pressureof the substance.

The apparatus further includes, wherein the trays are rotatable.

The apparatus further includes, wherein the desiccant is selected fromthe group consisting of lithium chloride, calcium chloride, calciumsulfate, phosphorous pentoxide, triethylene glycol, diethylene glycol,ethylene glycol, tetrathylene glycol, silica gel, and molecular sieves.

The apparatus further includes a plurality of fans within the processingchamber for circulating the environment therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with features, objects, and advantages thereof may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 is a phase diagram.

FIG. 2 is a diagrammatic front perspective view of an apparatus for thecontinuous sublimation of a substance in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION

In describing the preferred embodiments of the invention illustrated inthe drawings, specific terminology will be used for the sake of clarity.However, the invention is not intended to be limited to the specificterms so selected, and it is to be understood that each specific termincludes all technical equivalents that operate in a similar manner toaccomplish a similar purpose.

FIG. 1 shows an example of an apparatus 100 for the continuoussublimation of a substance such as water in a lyophilization process inaccordance with one embodiment of the present invention. As shown, ahollow processing chamber 102 forming the sublimation chamber iscylindrically or polygonally enclosed by sidewall 104 which extendsaround the circumference of the chamber, a top plate 106, and a bottomplate 108. The chamber has a plurality of internal processing zoneswhich are contiguous with each other thereby forming essentially asingle continuous processing chamber where sublimation of a substance toform a sublimate takes place simultaneously or serially within thechamber at substantially atmospheric conditions. The chamber 102 ispreferably maintained at atmospheric pressure, however, a slightpressure of about ±0.10 inches water, and more preferably, about ±0.05inches water is also contemplated.

The apparatus 100 can operate continuously by continuously supplyingmaterial to be processed through a feed port 112 such as in the topplate 106 and removing continuously the exhaust gas which includes thesublimate from a vapor outlet 114 such as also in the top plate orsidewall 104. The lyophilized material, referred to as freeze driedproduct, may be removed from the apparatus 100 through a productdischarge port 116 such as in the bottom plate 108. The variousprocessing zones typically operate at substantially the same atmosphericpressure and substantially the same temperature, or one zone may operateat a higher or lower temperature relative to other zones if desired.

The processing zones within the chamber 102 may be heated using recycledheated air or inert gas such as nitrogen from heater 118 and intake fan120 supplied to the chamber through gas inlet 121. The heated gas mayalso be supplied to multiple levels within the chamber 102 as shown byheated gas inlets 121, 122, 125 connected to the heater 118. Separateheaters for each of the gas inlets may also be provided to providedifferent temperatures. Accordingly, the supplied heated gas may be atthe same or different temperatures for one or more of the processingzones. Generally, it is contemplated that heated air will be the heatingmedium for the sublimation process. However, heated nitrogen is thepreferred heating medium when oxidation within the processing chamber102 is a concern, other inert gases may also be used. In addition,electric or gas fire heaters may be used to heat gases as may bedesired.

The apparatus 100 includes any of a variety of components fortransferring the material through the different zones of the processingchamber 102. For example, the apparatus may incorporate a plurality ofvertically displaced material supports such as trays 130. According toone embodiment, the trays may include apertures (not shown), therebyallowing material to pass through or cascade downwardly from one tray toa lower tray through the processing chamber 102. For example, the trays130 may be attached to a rotating structure 132, and thus may rotateabout a substantially vertical axis as the structure rotates. Acantilevered device 134 can be provided extending over each of the traysto push material through the aperture as the trays rotate. Thecantilevered devices 134 may be constructed as wiper arms to transferthe material from one tray level to the next tray level below, orgyrating trays with large perforations may be used to shake the materialfrom one tray level down to the next tray.

The apparatus 140 further includes at least one drive assembly 136underlying the structure 132 which powers operations within the chamber102. The plurality of spaced apart stacked trays 130 are rotated by thedrive assembly 136. Alternatively, the trays 130 may remain stationary,and the cantilevered devices 134 may sweep across the trays totransition the material thereon from one tray to the next. Accordingly,the material being processed is transferred from the feed port 112 ontothe first tray level, and continuously through the processing chamber102 via the tray levels to the product discharge port 116.

Inside the chamber 102, the apparatus incorporates the vertical set oftrays 130 surrounding a centrally arranged set of vertically-alignedfans 140 connected to a rotatable fan shaft 142 coupled to the driveassembly 136. The fans 140 may be connected to the fan shaft 142 by keys144. The fans circulate the heated air or gases inside the chamber overthe material in each of the trays 130 to provide a uniform temperaturedistribution as the material is progressively transferred to lower traylevels. Each tray is connected to at least one stanchion 146, whereinseveral stanchions are positioned around the fan shaft 142, therebyforming the rotating structure 132 in the nature of a squirrel cage.According to this embodiment, the trays 130 are arranged as a rotatingtray structure which surrounds the fan shaft 142. Drive gears (notshown) cause the structure 132 to rotate, thereby causing the trays 130to revolve within the chamber 102.

The cantilevered devices 134 which form tray wipers may be positionedabove each tray 130, although not shown for each tray. As each trayrotates, the tray wiper transfers the supported material downwardly tothe next tray level. A rigidly mounted leveler 150 may also brush acrossthe top surface of the material placed within a tray, thereby levelingthe material and exposing material underneath the top portion to theenvironment within the chamber. Material that is spilled by the traywiper over the side of the tray (i.e., between the chamber sidewall 102and the rotating trays) falls onto catch plate 152. The catch plate,angularly positioned with respect to the trays 130, causes the materialwhich is spilled off a tray above to fall into a tray below. In thismanner, the material being processed cascades downwardly from the uppertray to the lower tray through the apertures or by the catch plate.According to one aspect, a turntable sweeper 148 may be positionedwithin the bottom of the processing chamber 102. The turntable sweepermay prevent complications potentially caused by material falling ontothe processing chamber bottom.

As the processed material is being rotated and moved as described abovebetween the trays, further heating elements may be implemented withinthe chamber 102. The several fans 140 within the chamber facilitatecirculation of the heated gas to effect a more even temperature profilein each zone within the chamber by pushing the internal environmentwithin the chamber across each of the trays 130. The trays 130 and fans140 are driven by the drive assembly 136.

Alternatively or additionally, internal heating within the chamber maybe used. For example, electrical heaters 170 may be placed within thechamber 102 at selected locations to heat the internal gas. In otherunits, U-tubes (i.e., hollow tubes with flames inside) may be positionedwithin the chamber and connected to an exhaust and a natural gas inletport. To prevent the heated gas within the chamber 102 from escaping,seal assemblies may be placed around the shaft 142 and near the openingin the top plate 106 and bottom plate 108.

According to one aspect, continuous sublimation may be performed in acommercial TurboDryer® system as modified pursuant to the presentinvention. However, other systems which may be used include any type ofa vertical apparatus with trays or plates or hearths that retain thematerial and in which the material moves down through the apparatus bymeans of arms, blades, or other such devices. Certain aspects of theconstruction of the apparatus described is disclosed and described inco-pending application Ser. No. 11/975,144, filed on Oct. 17, 2007, inco-pending application Ser. No. 12/456,427, filed on Jun. 15, 2009, andin co-pending application Ser. No. 12/589,394, filed Oct. 22, 2009,entitled “Method for the Pyrolytic Extraction of Hyrocarbon From OilShale”, the disclosures of which are incorporated herein by reference.

The sublimate and exhaust gas from the vapor outlet 114 are optionallypassed through a cooling device 124. The exhaust gas may be cooled ifdesired with water, air, or other fluid medium. Optionally, the exhaustgas may pass through a particulate filter 125 which may be arrangedbefore or after the cooling device 124.

In order to promote sublimation in the processing chamber, the partialpressure of the substance to be sublimed should be maintained less thanthe vapor pressure of the substance in the material being lyophilizedwhen frozen. By way of one example, this can be accomplished by passingthe exhaust gas from the processing chamber 102 through a desiccantsystem 128 to control the dew point before recycling the exhaust gasback to the processing chamber through the heater 118. Another way ofreducing the partial pressure of the substance to be sublimed in theprocessing chamber 102 is to introduce extremely dry gas such asextremely low dew point air or liquid nitrogen into the chamber.

The desiccant used in the desiccant system 128 may be either a soliddesiccant or a liquid desiccant. In the case of a solid desiccant,contact with the exhaust gas may be accomplished in any suitable meansknown to those skilled in the art, for example, a fluidized bed anddrying tubes containing particulate desiccant and the like. As theexhaust vapors pass over the desiccant, moisture within the exhaust gasis picked up by the desiccant thereby lowering the partial pressure ofthe water in the exhaust gas below its vapor pressure. Once thedesiccant has been spent, i.e., typically saturated with moisture, thedesiccant can be rejuvenated. In this regard, the spent desiccant can beheated to a temperature sufficient to evaporate the absorbed waterwhereby the desiccant can be reused. The recovered sublimate can berecycled for use in the initial freeze drying process.

In a continuous sublimation process, it is contemplated that a pluralityof desiccant systems 128 will be arranged in parallel with the exhaustgas discharged from the processing chamber 102. When one or more of thedesiccant systems becomes spent, it may be taken out of line while thedesiccant is being rejuvenated. The exhaust gas may therefore besupplied to a plurality of desiccant systems 128 using a manifold 172controlled by suitable valves to isolate individual desiccant systems asmay be desired during the lyophilization and desiccant regenerationprocesses.

In a similar manner, it is contemplated that the desiccant system 128can incorporate a liquid desiccant. Any number of known systems forcontacting the exhaust gas with a liquid desiccant can be used, forexample, a column where the liquid desiccant is fed to the top of thecolumn where it is contacted with the exhaust gas from the processingchamber. The liquid desiccant removes the moisture and is carried outthe bottom of the column. The dry exhaust gas leaves the top of thecolumn where it can be recycled back to the processing chamber 102through the intake fan 120 and heater 118. The liquid desiccant afterleaving the column can be fed to a heat exchanger for regenerating thedesiccant by removing the excess moisture. The regenerated desiccant canbe recycled back to the absorber column in a continuous cycle. Anyvariety of liquid or solid desiccants can be used in accordance with thepresent invention. For example, known desiccants include lithiumchloride, calcium chloride, calcium sulfate, phosphorous pentoxide,triethylene glycol, diethylene glycol, ethylene glycol, tetrathyleneglycol, silica gel, and molecular sieves.

The apparatus and method of the present invention has particularapplication in the lyophilization, i.e., freeze drying of variousmaterials which contain water. These materials can encompass a broadproduct line of food products and pharmaceutical compositions forpreserving the product as well as for reducing weight. It is to beunderstood that the apparatus and method of the present invention issuitable for use with any material containing a substance which iscapable of sublimation.

In accordance with one embodiment of the present invention, a foodproduct containing water is freeze dried using the apparatus and methodas described with respect to FIG. 2. The food product is initiallyprepared with a water content, either naturally or artificially. Thefood product is frozen by any suitable means to a temperature below 32°F. The frozen food product is reduced to particulate form in the rangeof about 8 to 200 mesh using any suitable grinding equipment. Theparticulate frozen food material is fed continuously to the processingchamber 102 onto the top tray 130 of the rotating trays via feed port112. The food product on each tray 130 rotates most of the way aroundthe interior of the processing chamber at each processing zone.

As each tray 130 rotates, the tray wiper 134 transfers the material tothe next underlying tray. The material that is spilled by the tray wipermay fall onto the catch plate 152 or other suitable device. The catchplate, angularly positioned with respect to the trays 130, passes thematerial which is spilled off the tray above to fall onto a tray below.In this manner, the food product being processed cascades downwardlythrough the processing chamber 102 from the top most tray to the bottommost tray.

As the food product continues down through the processing chamber 102,it is subject to an environment at substantially atmospheric pressure,and at a temperature which sublimation of the water in solid phaseoccurs, e.g., in the range of about 20°-30° F. The sublimation processcontinues by controlling the environment within the processing chambersuch that the partial pressure of the water in the chamber is less thanthe vapor pressure of the frozen water in the food material beingprocessed. As explained hereinabove, this is achieved by passing theexhaust gas through the desiccant system 128. The freeze dried materialis discharged from the processing chamber 112 via discharge port 116.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1-9. (canceled)
 10. A method for the continuous sublimation of asubstance, the method comprising: transferring material containing asubstance capable of sublimation through a plurality of processing zoneseach having a material support arranged in a vertical stack within aprocessing chamber; controlling an environment within the processingchamber at substantially atmospheric pressure and at a temperaturewhereby the substance sublimes to form a sublimate within theenvironment; and contacting the environment containing the sublimatewith a drying agent to control the partial pressure of the substancewithin the environment whereby the substance sublimes at thesubstantially atmospheric pressure at the temperature.
 11. The method ofclaim 10, wherein the drying agent comprises a desiccant.
 12. The methodof claim 10, wherein the drying agent comprises a gas having a dew pointwhereby the substance sublimes at substantially atmospheric pressure atthe temperature.
 13. The method of claim 10, further includingmaintaining the environment whereby the partial pressure of thesubstance in the processing zones is less than the vapor pressure of thesubstance in the material.
 14. The method of claim 13, further includingmaintaining the temperature in the range of about 20-30° F.
 15. Themethod of claim 11, including wherein the desiccant is selected from thegroup consisting of lithium chloride, calcium chloride, calcium sulfate,phosphorous pentoxide, triethylene glycol, diethylene glycol, ethyleneglycol, tetrathylene glycol, silica gel, and molecular sieves.
 16. Themethod of claim 10, further including supplying the material to theprocessing chamber in particulate form.
 17. The method of claim 16,further including supplying the material at a temperature of less than32° F. whereby the substance is frozen.
 18. The method of claim 10,further including separating the sublimate from the environment bycondensing the sublimate.
 19. A method for the continuous sublimation ofa substance, the method comprising: supplying material in particulateform containing a substance in a frozen state, the substance capable ofsublimation; cascading the material downwardly between a plurality oftrays vertically stacked within a processing chamber maintained undersubstantially atmospheric pressure; controlling the temperature of theenvironment within the processing chamber below 32° F. whereby thesubstance sublimes; and controlling the partial pressure of thesubstance within the environment within the process chamber less thanthe vapor pressure of the substance in the material at least bycontacting the environment with a desiccant or a gas having a low dewpoint, whereby the substance continuously sublimes within the processingchamber.
 20. The method of claim 19, further including supplying thematerial at a temperature of less than 32° F. whereby the substance iffrozen.
 21. The method of claim 19, wherein the substance compriseswater.
 22. The method of claim 19, further including circulating theenvironment within the processing chamber by a plurality of fans withinthe processing chamber.
 23. The method of claim 19, further includingseparating the sublimate formed from sublimation of the substance withinthe process chamber from the desiccant.
 24. The method of claim 19,further including separating the sublimate formed from sublimation ofthe substance within the processing chamber from the environment. 25.The method of claim 19, further including maintaining the temperature isin the range of about 20-30° F.
 26. The method of claim 19, wherein thedesiccant is selected from the group consisting of lithium chloride,calcium chloride, calcium sulfate, phosphorous pentoxide, triethyleneglycol, diethylene glycol, ethylene glycol, tetrathylene glycol, silicagel, and molecular sieves.
 27. The method of claim 19, further includingmaintaining the pressure between about ±0.10 inches of water.
 28. Amethod for continuous lyophilization by sublimation of a substance, themethod comprising: freezing a material containing a substance capable ofsublimation; providing the frozen material in particulate form to aprocessing chamber; cascading the frozen material downwardly within theprocessing chamber between a plurality of trays vertically stackedwithin the processing chamber, each of the trays forming a processingzone; maintaining a substantially atmospheric environment within each ofthe processing zones and at a temperature less than 32° F., whereby thesubstance sublimes forming a sublimate within the environment; andcontrolling the partial pressure of the substance within the environmentat each of the processing zones less than the vapor pressure of thesubstance in the frozen material.
 29. The method of claim 28, whereinthe substance comprises water.
 30. The method of claim 28, furtherincluding maintaining the environment between about ±0.10 inches water.31. The method of claim 28, further including condensing the sublimateto recover the substance.
 32. The method of claim 28, wherein thecontrolling includes supplying nitrogen, low dew point gas or dry gasinto the processing chamber for controlling the partial pressure in theenvironment of the substance being sublimed; or contacting theenvironment comprising a drying atmosphere with a desiccant.
 33. Themethod of claim 28, wherein the controlling includes contacting theenvironment with a gas having a dew point whereby the substance sublimesat substantially atmospheric pressure at the temperature.
 34. The methodof claim 28, further including maintaining the temperature in the rangeof about 20-30° F.
 35. A method for continuous lyophilization bysublimation of water in a solid phase within a material, the methodcomprising: freezing a material containing water to a temperature below32° F., whereby the water is present in a solid phase; providing thefrozen material in particulate form in the range of about 8-200 mesh toa processing chamber; cascading the frozen material in particulate formdownwardly within the processing chamber between a plurality of rotatingtrays vertically stacked within the processing chamber, each of thetrays forming a processing zone; maintaining a substantially atmosphericenvironment in the range of about ±0.10 inches water within each of theprocessing zones and at a temperature in the range of about 20-30° F.,whereby the water in a solid phase sublimes forming a sublimate withinthe environment; controlling the partial pressure of the water withinthe environment at each of the processing zones less than the vaporpressure of the water in solid phase in the frozen material by at leastcontacting the environment with a desiccant; heating the environmentafter contacting the environment with the desiccant; and recycling theheated environment to the processing chamber.
 36. The method of claim35, further including condensing the sublimate from the environment. 37.An apparatus for continuous sublimation of a substance, the apparatuscomprising: a processing chamber containing a substantially atmosphericenvironment, the processing chamber having a plurality of traysvertically stacked therein for cascading therethrough a materialcontaining a substance capable of sublimation to form a sublimate at apredetermined temperature; and a desiccant in fluid communication withthe processing chamber for maintaining the partial pressure of thesubstance within the environment within the processing chamber less thanthe vapor pressure of the substance.
 38. The apparatus of claim 36,wherein the trays are rotatable.
 39. The apparatus of claim 37, furtherincluding a plurality of fans within the processing chamber forcirculating the environment therein.
 40. The apparatus of claim 36,wherein the desiccant is selected from the group consisting of lithiumchloride, calcium chloride, calcium sulfate, phosphorous pentoxide,triethylene glycol, diethylene glycol, ethylene glycol, tetrathyleneglycol, silica gel, and molecular sieves.